Solar chimney for daytime and nighttime use

ABSTRACT

A solar chimney includes a solar collector which heats air in the chimney, producing updrafts which can be harnessed to perform useful work. The solar collector may be located inside or outside the chimney. The device may include a reservoir for storing a heat transfer fluid, thus enabling the device to be used during both day and night. The solar chimney may be equipped with both internal and external solar collectors, operating in parallel. The solar chimney may also be equipped with multiple external solar collectors, connected in parallel to a reservoir. The solar chimney of the invention provides a convenient and practical way to convert solar energy into electrical power.

BACKGROUND OF THE INVENTION

This invention relates to the production of electric power, or otheruseful work, from solar energy.

In a solar chimney, energy from the sun heats the air in an elongated,enclosed vertical structure, so as to create an updraft in thestructure. The moving air can then be used to drive a turbine to produceelectric power, or it can be made to perform other useful work. A solarchimney has the advantage that it does not depend on the presence ofwind, but requires only sunlight as the source of energy.

Examples of solar chimneys, as shown in the prior art, are given in U.S.Pat. Nos. 3,979,597, 4,275,309, 4,331,042, 4,433,544, 5,381,048,6,016,015, 6,089,021, 6,772,593, and 7,026,723, the disclosures of whichare incorporated by reference herein.

The present invention provides improvements over the solar chimneys ofthe prior art. The solar chimney of the present invention concentratesincident solar radiation into a high-intensity beam which can bedirected or focused onto a collector. In one embodiment, the system ofthe present invention can be used to generate power even duringnighttime.

SUMMARY OF THE INVENTION

In a first embodiment, the solar chimney of the present inventioncomprises a solar collector located within the chimney. Solar radiationfrom outside the chimney is concentrated by a reflector, or itsequivalent, and passes through an aperture in the chimney wall. Theaperture may include a lens, or the lens may be omitted. Theconcentrated solar radiation impinges on a collector which distributesabsorbed heat to a heat exchanger, also located within the chimney. Airin the chimney is heated by convection, due to the temperaturedifference between the heat exchanger and the surrounding air. Theheating of the air produces an updraft in the chimney. The updraftcomprises a stream of moving air which can be used to perform usefulwork.

The heat exchanger could be a fixed structure, or it could comprise aplurality of vanes which rotate relative to the chimney, either underthe power of a motor, or by free rotation under the influence of theupdrafts in the chimney.

In another embodiment, the solar collector is located outside thechimney, while the heat exchanger is located inside the chimney. Heatabsorbed by the collector is conveyed to the heat exchanger, either witha solid heat conductor, or by a heat transfer fluid.

In another embodiment, the solar chimney includes a reservoir having twoor more compartments, the system being usable during both day and night.During the day, heat transfer fluid is pumped from the reservoir, andinto a solar collector, where the fluid absorbs heat from solarradiation. The heated fluid is then conveyed into a heat exchangerlocated within a chimney. The fluid, which has given up some of its heatto the heat exchanger, but which is still relatively hot, is returned tothe reservoir, and is stored in a different compartment from the onefrom which the fluid was initially withdrawn. During the night, heattransfer fluid is conveyed directly from the reservoir into the heatexchanger in the chimney. Thus, an updraft in the chimney can beproduced even during the night. The compartments of the reservoir aredefined by one or more movable partitions which provide mechanical andthermal separation between compartments.

In another embodiment, the solar chimney may include both an internalsolar collector, located inside the chimney, and an external solarcollector, located outside the chimney and connected to transfer heat toa heat exchanger within the chimney. Both the internal collector and theexternal collector operate in tandem.

In still another embodiment, the solar chimney comprises multipleexternal solar collectors, the solar collectors providing heat to thefluids in a multiple-compartment reservoir, the system being usableduring both day and night.

The present invention therefore has the primary object of providing asolar chimney, in which solar energy heats air in the chimney, causingupdrafts which can be harnessed to perform useful work.

The invention has the further object of providing a solar chimney inwhich the solar collector and heat exchanger are located within thechimney.

The invention has the further object of providing a solar chimney inwhich a solar collector, external to the chimney, provides heat to aheat exchanger located within the chimney.

The invention has the further object of providing a solar chimney whichis capable of operation during both day and night.

The invention has the further object of providing a solar chimney havingboth internal and external solar collectors.

The invention has the further object of providing a solar chimney havingmultiple external solar collectors, the collectors being connected inparallel to feed a reservoir, the solar chimney being usable during bothday and night.

The invention has the further object of improving the efficiency of asolar chimney.

The invention has the further object of providing an improved device forharnessing the energy of the sun to do useful work.

The reader skilled in the art will recognize other objects andadvantages of the present invention, from a reading of the followingbrief description of the drawings, the detailed description of theinvention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a cut-away perspective view of a combined solarcollector and heat exchanger, together with means for concentratingsolar energy, made according to the present invention.

FIG. 2 provides another perspective view of the combined solar collectorand heat exchanger of FIG. 1.

FIG. 3 provides yet another perspective view of the combined solarcollector and heat exchanger of FIG. 1.

FIG. 4A provides a cross-sectional view, taken from the top, of thecombined solar collector and heat exchanger of FIGS. 1-3.

FIG. 4B provides a cross-sectional view, taken from the side, of thedevice of FIG. 4A.

FIG. 5 provides a cut-away perspective view of a combined solarcollector and heat exchanger, made according to the present invention,wherein solar energy enters the device through an aperture having nolens.

FIG. 6A provides a cross-sectional view, taken from the top, of thedevice shown in FIG. 5.

FIG. 6B provides a cross-sectional view, taken from the side, of thedevice shown in FIG. 5.

FIG. 7 provides a cut-away perspective view of another embodiment of thepresent invention, wherein the absorptive coating and target substraterotate relative to the body of the combined solar collector and heatexchanger.

FIG. 8A provides a cross-sectional view, taken from the top, of theembodiment shown in FIG. 7.

FIG. 8B provides a cross-sectional view, taken from the side, of theembodiment shown in FIG. 7.

FIG. 9 provides a cut-away perspective view of another embodiment of thepresent invention, in which the solar collector is external to achimney.

FIG. 10A provides a cross-sectional view, taken from the top, of theembodiment of FIG. 9.

FIG. 10B provides a cross-sectional view, taken from the side, of theembodiment of FIG. 9.

FIG. 11 provides a cut-away perspective view of another embodiment ofthe present invention, in which the solar collector is external to thechimney, and in which heat is exchanged through a heat transfer fluid.

FIG. 12 provides a cross-sectional view of the embodiment of FIG. 11.

FIG. 13A provides a cross-sectional view, taken from the side, of thesolar collector portion of the embodiment of FIG. 11.

FIG. 13B provides a cross-sectional view, taken from the top, of thesolar collector portion of the embodiment of FIG. 11.

FIG. 14 provides a schematic diagram of another embodiment of thepresent invention, this embodiment including a reservoir for storingheated fluid, for day/night operation.

FIGS. 15A, 15B, and 15C provide schematic diagrams illustrating theoperation of the system of FIG. 14 in daytime and nighttime, and showingthe change-over from one mode to the other.

FIG. 16 provides a partly schematic, and partly cross-sectional diagramshowing details of the reservoir used in the embodiment of FIG. 14.

FIG. 17 provides a schematic diagram of another embodiment of thepresent invention, in which internal and external solar thermalcollectors operate in tandem or individually.

FIG. 18 provides a schematic diagram of another embodiment of thepresent invention, wherein multiple solar collectors are used, inparallel, to feed a reservoir.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3 and 4A-4B illustrate an embodiment of the present invention inwhich a solar collector and heat exchanger are provided as a single unitlocated inside a generally cylindrical chimney. The chimney comprises anenclosure for the solar collector and heat exchanger. Solar reflector 1,located outside chimney 10, concentrates sunlight into a high-intensitybeam, which passes through lens 3. The lens, which is held by lensholder 4, serves as a means for allowing solar energy to enter thechimney. The lens refocuses the beam through aperture 5, located insidethe chimney. The aperture could be made of the same material as that ofthe chimney, or it could be made of special heat-resistant material.

The focused solar radiation impinges on absorptive coating 7 disposed ontarget substrate 9. The absorptive coating comprises a material whichfacilitates the absorption of solar energy. The target substrate is inthermal contact with primary fins 11, and is in indirect thermal contactwith secondary fins 13. The fins provide a heat exchange surface forheating convected air passing through the chimney. Energy transfer iseffected between the hot surface of the solar thermal collectors and thesurrounding air, due to the temperature differential.

Scattered incident solar radiation is absorbed by another absorptivecoating 23 located on the inside surface of inner casing 15. Such heatabsorption heats the inner casing, and thus also heats the secondaryfins 13. Radiation emitted by the inner casing is reflected back ontothe inner casing and secondary fins 13 by the reflective inner surface17 of the outer casing 19 of the chimney. Insulation layer 21 preventsloss of heat due to conduction.

The device can also be provided with a fusible wire 25, disposed alongthe periphery of the aperture of the chimney, which would cause thereflectors to be switched off in the event of mis-focusing of thehigh-intensity beam, thus providing the chimney with a safety devicecomprising an emergency shut-off mechanism.

The solar energy collected within the chimney heats the air in thechimney, creating an updraft which can then be used to drive a turbineto produce electricity, or to perform other useful work.

FIGS. 5, 6A and 6B illustrate another embodiment of the invention. Thisembodiment is similar to that of FIGS. 1-4, except that the solar beamis not focused by a lens. Instead, the solar radiation is simplydirected through an aperture 31 formed in the wall of the chimney. Theaperture may be fitted with a window 33, comprising a transparent ortranslucent material. The window thus allows solar energy to enter thechimney, while effectively sealing the enclosure by preventing air fromentering. The inner casing 35 also has an aperture 37. The othercomponents are similar to those of the embodiment of FIGS. 1-4, exceptthat the target substrate 39 is displaced, compared to its position inthe previous embodiment, due to the fact that the incoming beam is notfocused by a lens.

Both the embodiment with a lens and the embodiment without a lens areuseful, but the embodiment including the lens is considered preferable.A lens causes the incoming rays to diverge upon entering the chimney.The divergent rays then fall upon a larger area of the target substrate.The larger the area, the more energy the substrate can absorb, resultingin greater overall heating of the air in the chimney, and thus resultingin the production of more powerful updrafts. A lens also inherentlyprevents the intrusion of air into the chimney, so the lens thus servesboth the purpose of focusing the rays and of keeping air out of thechimney. The only disadvantage is that the lens is more expensive thanthe simple window. In these embodiments, both the lens and the windowcomprise means for allowing solar radiation to enter the enclosuredefined by the chimney.

Other means for directing solar energy into the chimney could be usedinstead of what is shown in the drawings. For example, sunlight could bedirected into the chimney by a series of mirrors, a series of lenses, orlight tubes, or some combination thereof.

Another embodiment of the invention is shown in FIGS. 7, 8A, and 8B. Inthis embodiment, the target substrate and its absorptive coating rotaterelative to the fixed chimney. Vanes 43 define the target substrate, thetarget substrate having an absorptive coating (not explicitly shown).The vanes are mounted for rotation on shaft 41. The other components aresimilar to the embodiment of FIGS. 1-4. This embodiment could also beapplied to the arrangement of FIGS. 5-6.

Rotation of the target substrate, in the embodiment of FIGS. 7-8, couldbe accomplished automatically, i.e. by allowing the structure to rotatefreely under the influence of the updraft in the chimney. Alternatively,rotation could be caused by a motor. Arrow 45, which indicates thedirection of rotation, is also intended to represent a motor for causingsuch rotation. The rotation of the vanes provides greater turbulenceinside the chimney, resulting in higher heat flux to the convected air.The solar collector could also be provided with a combination ofrotating and stationary elements.

FIGS. 9, 10A, and 10B show another embodiment of the invention, whereinthe solar collector is external to the chimney. A solar reflector 61directs a high-intensity solar beam through glass 63 held by windowframe 65, formed in container 67. The container is a sealed enclosure,and may be metallic or non-metallic. The container is depicted as acylinder, but it could have other shapes. The beam entering thecontainer then passes through lens 69. The focused solar radiationimpinges on absorptive coating 71 of target substrate 73. The targetsubstrate conducts heat, through a solid heat conductor 75 to a heatexchanger 77 located within chimney 79. The heat exchanger 77 issurrounded by a reflective casing 78 which reflects radiation emitted bythe heat exchanger. The reflective casing 78 is fitted with fins 74 toimprove the heat transfer. Chimney 79 may be insulated.

The container 67 includes an inner casing 81 which preferably has anabsorptive outer coating, for absorbing scattered incident solarradiation. The inner casing may also have a reflective inner surfacethat reflects scattered incident radiation back onto the absorptivecoating 71 of target substrate 73. The outer casing of the container mayalso include a reflective inner surface, for reflecting emittedradiation back to the inner casing. The container is preferablyevacuated and totally insulated to reduce or eliminate convective andconductive heat loss.

Solar energy directed onto the collector is thus converted into heat, inthe collector, transferred to the chimney, and then used to heat the airin the chimney, forming the desired updraft for performing useful work.

FIGS. 11-13 illustrate another embodiment of the invention, wherein thesolar collector is again external to the chimney, and wherein heat fromthe solar collector is transferred through a heat transfer fluid. Inthis embodiment, reflector 91 directs a high-intensity beam of solarradiation through window 93 of container 95. The container is preferablysealed and evacuated. The beam passes through aperture 97 inintermediate casing 99. The beam is then refocused by lens 101 ontoabsorptive coating 103 of target substrate 105. The target substrate isthereby heated.

Scattered incident solar radiation is absorbed by absorptive coating 107of inner casing 109, thereby heating the inner casing. Emitted radiationis absorbed by absorptive coating 111 disposed on the inside surface ofintermediate casing 99. The outer casing 113 has a reflective innersurface 118 which reflects radiation emitted by the intermediate casing99 back onto the intermediate casing. Heat transfer fluid coils 115,117, and 119 are attached to intermediate casing 99, inner casing 109,and target substrate 105, respectively. Heat transfer fluid enters thecontainer, first conveying thermal energy away from intermediate casing99, then from inner casing 109, and finally from target substrate 105.The heat transfer fluid coils are in fluid connection with the samefluid conduit which conveys the fluid between the solar collector andthe heat exchanger in the chimney. As the heat transfer fluid carriesthe heat, it becomes progressively hotter, and leaves the collector atits highest temperature before transferring thermal energy to ambientair at the heat exchanger within chimney 121.

The heat transfer fluid may be circulated by a pump (not shown), or by apassive means, such as a thermosiphon, wherein liquid circulates in avertical closed-loop system, without a pump, due to natural convection.

As in the previous embodiments, the thermal collectors can be eithermetallic or non-metallic. They are preferably evacuated and totallyinsulated to reduce or eliminate convective and conductive heat loss.

FIG. 14 illustrates an embodiment of the invention suitable for use inboth daytime and nighttime operation. FIG. 14 illustrates the generaloperation; specifics are shown in FIGS. 15A-C. In brief, during thedaytime, heat transfer fluid is pumped, by pump P1, from reservoir 201,through valve V1, and into external solar collector 203. The heattransfer fluid receives heat in the solar collector, and then flows outof the collector and into a heat exchanger located inside chimney 205.The heat transfer fluid gives up heat, in the chimney, to the airsurrounding the heat exchanger, and then flows back to the reservoir.During the nighttime, valve V1 is closed and valve V2 is opened, causingstored heated fluid from the reservoir to flow directly into thechimney, where heat from the fluid is transferred to surrounding air inthe same manner. The fluid then returns to the reservoir.

Thus, the valves, conduits, and pump together comprise means fordirecting heat transfer fluid between the reservoir, the solarcollector, and the heat exchanger in the chimney.

FIGS. 15A-C illustrate the operation of the arrangement of FIG. 14 inmore detail. Consider first the operation during the day, illustrated inFIG. 15A. In this case, valve V1 is open, valve V2 is closed, and valveV3, the outlet valve of the reservoir, is open.

Heat transfer fluid is heated by absorption of solar radiation incollector 301. The heated fluid releases the absorbed energy in chimney303, which is assumed to include a heat exchanger, as described above.The energy released in the chimney is a function of the temperaturedifferential between the heated fluid and ATD, the air temperatureduring daytime. After passing through the chimney, the fluid hastemperature LTD, which is defined as the fluid temperature after heatexchange with daytime air. The temperature LTD is still high enough totransfer residual energy to cooler nighttime air, the temperature of thenighttime air being designated as ATN. In other words, nighttime air canbe heated by the fluid that has already been used to heat daytime air,enabling the chimney to generate power during the night.

It is crucial that the fluids of different temperatures, enabling thisoperation, be prevented from mixing. Partition 305 acts both as aphysical and thermal barrier.

FIG. 15A represents the status of the reservoir at the start of the day.The upper portion of the reservoir holds heat transfer fluid having atemperature LTN, which indicates the temperature of the fluid after heatexchange with nighttime air. This fluid is withdrawn from the reservoir,through valves V3 and V1, and conveyed to collector 301, where it isheated. The fluid then flows to chimney 303, where it releases part ofits absorbed energy. The hot fluid heats the air in the chimney havingtemperature ATD, creating an updraft in the chimney, the updraft beingsufficiently powerful to perform useful work.

The system described is a closed system, so as the fluid (at temperatureLTN) is withdrawn from the upper portion of the reservoir, an equalamount of heat transfer fluid, having temperature LTD, is simultaneouslyfilling the bottom portion of the reservoir. Such fluid accumulates inthe lower portion of the reservoir, throughout the day. The partition305 adjusts itself automatically, being pulled up by the diminishingvolume of fluid in the upper portion, and being pushed up by theincreasing volume of fluid in the lower portion. Expansion bellow 307provides pressure relief as the partition moves upward.

As the day progresses, partition 305 moves upward within the reservoir.When the day ends, the barrier will be at or near the top of thereservoir, as shown in FIG. 15B. At this point, the reservoir is largelyfilled with fluid which has released its heat to daytime air, and whichhas temperature LTD. It is now time to change over to nighttimeoperation.

The partition 305 must be moved down to prevent the incoming heattransfer fluid, which will have released heat to nighttime air and whichwill have temperature LTN, from mixing with the hotter fluid which hasreleased heat to daytime air (LTD). The change over is accomplished byopening valve VR, which temporarily renders the partition ineffective.The partition can then be lowered to the bottom of the reservoir, eitherby adjusting its density to make it heavier, or by mechanical means.After the partition has been lowered, valve VR is again shut, restoringthe physical and thermal barrier between fluids of differenttemperature. Valve V1 is then shut, and valve V2 is opened, so thatfluid from the reservoir can flow directly to the chimney 303, bypassingthe solar collector 301. The solar collector is, of course, unnecessaryand useless at night.

The opening of the valve VR causes the fluid in the reservoir to becomefluid having temperature at or near LTD. This fluid will flow intochimney 303, and can release its residual energy to the cooler nighttimeair, having temperature ATN, to generate power. By dawn, the fluid inthe bottom portion of the reservoir will be fluid having temperatureLTN, i.e. fluid which has released heat to nighttime air. That is whythe upper and lower portions of the reservoir are designated by LTN andLTD, respectively, in FIGS. 15A and 15B, but are designated by LTD andLTN, respectively, in FIG. 15C.

At dawn, the fluid having temperature LTN will have replaced the fluidof temperature LTD, in the lower portion of the reservoir. The valve VRis then opened to lower the partition again. Valve V1 is then opened,and valve V2 is closed, so that the daytime process can begin again.

The reservoir could be provided with multiple partitions and/or multipleinlet and outlet valves. The use of multiple partitions enables moredifferentiation among fluids of different temperatures, resulting ingreater efficiency and more constant energy output produced by theupdrafts formed in the chimney. Thus, the present invention uses areservoir having two or more compartments, the compartments beingdefined by the partitions.

FIGS. 15A-C thus show how fluids of different temperatures can be keptseparated to prevent mixing. The process requires that mixing beprevented. Switching of the operation of the system is effected simplyby opening the valve VR. The process begins again when valve VR isclosed.

FIG. 16 provides further details of the reservoir represented in FIGS.14 and 15A-C. This figure also illustrates the case in which there maybe multiple partitions and multiple outlet valves. In FIG. 16, there areshown partitions 321, 323, and 325. Disposed between the levels of thesepartitions are valves V3A, V3B, and V3C, the latter valves correspondingto outlet valve V3 shown in FIGS. 15A-C. FIG. 16 also shows, in adetailed view, the movement of the partitions relative to the wall ofthe reservoir. As shown in the figure, the partition can be mounted torollers 331 which move within track guide 333.

The reservoir can be metallic or non-metallic, and can be disposedhorizontally or vertically. The reservoir could be uninsulated, althoughit is preferred to provide insulation. The reservoir could be locatedeither above ground or below ground. The expansion bellow accommodatesthermal expansion of the heat transfer fluid stored therein. Thereservoir is totally enclosed, forming a closed-loop system with thesolar thermal collector.

The partition or partitions are thermally insulated. The locations wherethe partition meets the wall of the reservoir constitutes a seal,creating both a physical and a thermal barrier to fluid on either sideof the partition. The partition can thus move according to the change involume of the fluid contained within the space bounded by the partition.

The sealing between the reservoir wall and the partition is provided bya mechanical seal or an elastomer. The partition is preferably equippedwith a mechanism, which could be mechanical, electrical, or physical,that can move the partition to a desired position.

FIG. 17 shows another embodiment of the present invention, in which thesystem includes both an internal and an external solar collector. Asindicated in the figure, chimney 401 includes internal solar collector403 and external solar collector 405. Thus, the figure shows a combinedsolar collector and heat exchanger, located entirely within the chimney,and a heat exchanger, spaced apart from the combined solar collector andheat exchanger, used in connection with the external solar collector.These solar collectors can be constructed as described in the previousembodiments.

With regard to the external solar collector, FIG. 17 shows the heatbeing transferred by a heat transfer fluid, but it is understood thatthis embodiment could include the use of a solid heat transfer device,as described earlier.

The solar collectors thus work in tandem, both generating heat withinthe chimney, and thereby producing updrafts which can perform usefulwork. The use of some or all of the features of the previousembodiments, working in tandem, increases the output of the solarchimney. A reservoir could also be added to the arrangement of FIG. 17,in the same manner described in previous embodiments, so that power canbe generated both during the day and during the night.

FIG. 18 shows another embodiment of the invention. In this embodiment,there are multiple solar collectors, connected in parallel, to feed areservoir. The reservoir is constructed in the same manner describedearlier. FIG. 18 shows that the reservoir can be supplied by one or moresolar collectors. For simplicity of illustration, the connection(s)between the reservoir and a heat exchanger located within the chimneyare not shown, but it is understood that FIG. 18 comprises amodification of the embodiment shown in FIGS. 14-16. The use of multiplesolar collectors increases the security and flexibility of the system.The system could be expanded by adding one solar collector at a time,without disrupting the operation of the system. Also, the use ofmultiple solar collectors connected in parallel makes it practical todisconnect one collector for repair, without halting operation of theoverall system.

The construction of the solar chimney is designed to minimize heat loss.The solar chimney can be made of brick, concrete, fiberglass, steel, orother materials, or combinations of the above, consistent with therequirements of maximum economy, high strength, low heat loss, andefficient air flow. The chimney can be constructed of pipes made offiberglass, which could be single-walled or multi-walled, and whichcould be made with or without stiffeners such as a honeycomb, and withor without insulation. The pipes can be lined or unlined, and could beof the same or different materials, consistent with the requiredtemperatures of operation.

It should be understood that features described with respect to aparticular embodiment are often applicable to other embodiments. Forexample, the rotatable vanes of the heat exchanger can be provided bothin the case where the solar collector is located within the chimney, andin the case where the solar collector is outside the chimney. The solarcollectors of both embodiments could be provided with either a lens, awindow, or an aperture for allowing solar radiation to enter. In thecase where the solar collector is inside the chimney, the lens or windowor aperture would be located in the chimney wall. In the case where thecollector is outside the chimney, the lens or window or aperture wouldbe located in a housing for such collector.

Similarly, in cases in which an external solar collector is used,without a reservoir, the means of heat transfer between the solarcollector and the heat exchanger inside the chimney could be either asolid heat conductor or a heat transfer fluid. For embodiments with areservoir, it is necessary that the heat transfer medium be a fluid.

In the embodiment of FIG. 17, the external solar collector couldtransfer heat to the inside of the chimney by a solid heat conductor, asshown in FIG. 9, instead of the heat transfer fluid as suggested in FIG.17.

Although a reservoir is not shown in FIG. 17, a reservoir could be usedwith any or all of the external solar collectors used in thatembodiment, in the same manner disclosed in FIGS. 14-16.

Thus, the appearance of a particular feature in a particular embodimentshould not be interpreted to limit the use of such feature to thatembodiment. On the contrary, the disclosed features may, when logicaland appropriate, be combined in many different ways.

The reader skilled in the art will recognize other variations of theinvention. Such variations should be considered within the spirit andscope of the following claims.

1. A solar chimney, comprising: a) an enclosure defining a path for flowof a fluid to be heated, the enclosure containing a heat exchanger, b) asolar collector located outside the enclosure, c) a reservoir forstoring a heat transfer fluid, and d) means for directing heat transferfluid between the reservoir, the solar collector, and the heatexchanger, wherein, in a first position of the directing means,appropriate for use during daytime, heat transfer fluid can flow fromthe reservoir, then into the solar collector, then into the heatexchanger, and then back to the reservoir, and wherein, in a secondposition of the directing means, appropriate for use during nighttime,heat transfer fluid can flow from the reservoir, into the heatexchanger, and then back to the reservoir.
 2. The solar chimney of claim1, wherein the reservoir includes a movable partition which thermallyand mechanically divides the reservoir into compartments.
 3. The solarchimney of claim 2, wherein the directing means is configured such thatheat transfer fluid is withdrawn from a first compartment of thereservoir, and such that heat transfer fluid is returned, from the heatexchanger, into a different compartment of the reservoir.
 4. The solarchimney of claim 1, wherein the solar collector comprises a targetsubstrate located within a housing, the housing including means forallowing solar radiation to enter the housing and to impinge upon thetarget substrate, the target substrate being in thermal contact with aportion of the directing means so as to transfer heat to fluid beingconveyed by the directing means.
 5. The solar chimney of claim 4,wherein the target substrate is located within an inner casing disposedwithin the housing.
 6. The solar chimney of claim 5, wherein there is anintermediate casing surrounding the inner casing and located within thehousing.
 7. The solar chimney of claim 6, wherein the inner casing has aheat absorptive inner surface, wherein the intermediate casing has aheat absorptive inner surface, and wherein the housing has a reflectiveinner surface.
 8. The solar chimney of claim 1, wherein the reservoirincludes at least two movable partitions, the partitions comprisingmeans for thermally and mechanically dividing the reservoir intocompartments.
 9. The solar chimney of claim 2, wherein the partition ismounted to rollers which are movable within a track guide formed in awall of the reservoir.
 10. The solar chimney of claim 1, wherein thedirecting means include conduits for directing heat transfer fluid fromthe reservoir and into either of the solar collector or the heatexchanger, valves for opening and closing some of said conduits, and apump for conveying heat transfer fluid through the conduits.
 11. Thesolar chimney of claim 1, wherein there are a plurality of solarcollectors located outside the enclosure, and wherein all of said solarcollectors are connected to said reservoir.
 12. A solar chimney,comprising: a) an enclosure defining a path for flow of a fluid to beheated, the enclosure containing a heat exchanger, b) a plurality ofsolar collectors located outside the enclosure, c) a reservoir forstoring a heat transfer fluid, and d) means for directing heat transferfluid between the reservoir, the solar collectors, and the heatexchanger, wherein, in a first position of the directing means,appropriate for use during daytime, heat transfer fluid can flow fromthe reservoir, then into at least one of the solar collectors, then intothe heat exchanger, and then back to the reservoir, and wherein, in asecond position of the directing means, appropriate for use duringnighttime, heat transfer fluid can flow from the reservoir, into theheat exchanger, and then back to the reservoir.
 13. The solar chimney ofclaim 12, wherein the reservoir includes a movable partition whichthermally and mechanically divides the reservoir into compartments. 14.The solar chimney of claim 12, wherein the directing means is configuredsuch that heat transfer fluid is withdrawn from a first compartment ofthe reservoir, and such that heat transfer fluid is returned, from theheat exchanger, into a different compartment of the reservoir.
 15. Amethod of operating a solar chimney system, the system including achimney having a heat exchanger disposed therein, a solar collector, anda reservoir, the heat exchanger, solar collector, and reservoir beinginterconnected by a plurality of conduits and valves, the methodcomprising: a) directing a heat transfer fluid, during daytime, from thereservoir, into the solar collector, and into the heat exchanger, andreturning the heat transfer fluid to the reservoir after the fluid hasreleased some heat to the heat exchanger, thereby causing air in thechimney to become heated and to move by convection, b) directing theheat transfer fluid, during nighttime, from the reservoir to the heatexchanger, and returning the heat transfer fluid to the reservoir afterthe fluid has released some heat to the heat exchanger, thereby causingair in the chimney to become heated and to move by convection, c) usingmoving air in the chimney to perform useful work.
 16. The method ofclaim 15, wherein the reservoir has a plurality of compartments definedby at least one movable partition disposed within the reservoir, andwherein fluid is withdrawn from one of said compartments, and whereinfluid is returned to another of said compartments.
 17. The method ofclaim 15, wherein step (b) is preceded by the step of closing a valve ina conduit connecting the reservoir to the solar collector, and opening avalve in a conduit connecting the reservoir to the heat exchanger. 18.The method of claim 15, wherein steps (a) and (b) are performed inrepeated alternating fashion, wherein the solar chimney system isoperated substantially continuously, during day and night.
 19. Themethod of claim 15, wherein the reservoir has at least threecompartments defined by at least two partitions, and wherein the heattransfer fluid is withdrawn from a plurality of said compartments. 20.The method of claim 16, further comprising moving a position of at leastone partition before switching from daytime operation to nighttimeoperation and vice versa.